THERE'S ALWAYS A REASON

About those inner walls at TMI: they are contaminated, not activated.

Radioactivity can spread in two ways: by transportation or induction. The small amount of radioiodine that reached the US in rain water after the Chernobyl accident was iodine that 10 days earner had actually been in Chernobyl. It was imported by the natural circulation of the atmosphere, and washed down by the rain. This type of transportation results in contamination.

The other way is activation: an originally stable atom can be induced to become radioactive under circumstances arising only near the location of a nuclear reaction, namely when a neutron (or other particle) is absorbed by the nucleus of the originally stable atom. This is induced radioactivity or activation.

The difference is important for seeing through the scare stories by the opponents of both civil defense (fallout does not activate!) and food irradiation.